Method of producing an intense, high-purity k x-ray beam

ABSTRACT

This method comprises the steps of bombarding an X-ray target of atomic number Z with a substantially monoenergetic electron beam having an energy in the region from about (Z+0.0014Z3) to about (3Z+0.0092Z3) kev. and collimating into an output beam the X-rays emitted from the target at an angle in the range from about 120* to about 180* with respect to the direction of the electron beam.

United States Patent 1 [1 1 3,752,989

Motz et a]. 1 Aug. 14, 1973 METHOD OF PRODUCING AN INTENSE, 3,567,928 3/1971 Davies et al. 313 55 x l-llGH-PURITY K X-RAY BEAM OTHER PUBLICATIONS Inventors: Joseph W. Motz, Charles E. Dick,

Robert C. Placious, Julian H. Sparrow, all of Rockville, Md.

represented by the Secretary of Commerce Filed: Jan. 27, 1972 Appl. No.: 221,249

US. Cl ..is0/401,'313/55 Int. Cl. l-lolj 37/00 Field of Search 250/84, 313/55 References Cited UNITED STATES PATENTS 8/1968 Brown et al. 250/90 X Applied X-rays, Fourth Edition, by G. L. Clark, Published by McGraw-Hill Book Co., New York, 1955, pages 128, & 151. QC 481C47 Primary Examiner-William F. Lindquist Attorney-David Robbins et a1.

[5 7 ABSTRACT This method comprises the steps of bombarding an X-ray target off atomic number Z with a substantially monoenergetic electron beam having an energy in the region from about (Z+0.0014Z-'*) to about (3Z+0.0092Z kev. and collimating into an output beam the X-rays emitted from the target at an angle in the range from about 120 to about with respect to the direction of the electron beam.

4 Claims, 4 Drawing Figures rmmmmuma 3752.989

sum 1 or 2 K XRAYS (ISOTROPIC) BREMSSTRAHLUNG HIGH -ENERGY auacmou BEAM THKN X-RAY TARGET COLLIMATING ANGLE RANGE PAIENTEBumm 3.752.989

SHEET 2 [IF 2 I I I I I I I PERCENT z BEAM PURITY 0 1 I I I I I 1 I I '2 ID E I I 1 I l I I I I E 3 K MAY YIELD MKSz-ELELML I0. I I

a l I I I i l I I I 10 KILOVOLTAGE 1 v ke 102 E E 180 DEGREES THICK TARGET I0 I I 1 I o 10 2.0 so 40 so ATOMIC NUMBER OF TARGET METHOD OF PRODUCING AN INTENSE, I-IIGH-PURITY K X-RAY BEAM BACKGROUND OF THE INVENTION This invention relates to a method of producing intense K X-ray beams which are minimally contaminated with background radiation or bremsstrahlung. These intense monoenergetic X-ray beams are useful in many X-ray applications such as medical diagnostics, fluorescence analysis, and calibration of high-flux detectors.

K X-rays are produced when K-shell electrons are removed from target atoms and electrons from higherenergy shells fall into the vacancies. The K-shell electrons may be removed by exciting the target with photons, atomic ions, or electrons. The photon and atomicion excitation methods produce relatively pure K X-ray beams, but the beam intensities are relatively low owing to the inefficiency of the fluorescence method and the unavailability of high-ion-current sources, respectively. The electron excitation method can produce high K X-ray intensities, but the previous methods have given rise to X-ray beams seriously contaminated with bremsstrahlung. The bremsstrahlung is produced when the incident electrons lose energy to the strong electric fields surrounding the target nuclei. The bremsstrahlung energy spectrum ranges from zero up to the kinetic energy of the incident electrons, whereas the dominant K X-ray energy is a fixed value, charac teristic of the target material.

SUMMARY OF THE INVENTION In the present invention, the maximum yields and minimum contamination of electron-excited K X-rays are obtained by using electrons with energies that are large compared to the atomic K-shell binding energies and by taking off the X-rays in the backward direction where the bremsstrahlung intensity is relatively low.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a polar plot of the intensities of the K Xrays and bremsstrahlung emitted from a thin target bombarded with high-energy electrons;

FIG. 2 is a cross section of an apparatus constructed in accordance with the principles of this invention;

FIG. 3 is a cross section of an alternativeapparatus;

FIG. 4 is a plot of the K X-ray beam purity and yield as a function of target atomic number and electron beam energy.

DETAILED DESCRIPTION OF THE INVENTION In accordance with this invention an X-ray target mounted in a suitable, evacuated chamber is bombarded with a substantially monoenergetic, high-energy electron beam and the X-rays emitted in the backward direction are collimated into an output beam.

In general the output beam is an intense high-purity K X-ray beam. The K X-ray beam intensity and purity are measured with a conventional X-ray spectrometer and are defined as follows: the K X-ray beam intensity or yield is the number of K X-rays emitted from the target per steradian per second, and the K X-ray beam purity is the ratio of the number of K X-rays to the total photon number integrated over the energy spectrum of the beam.

The output-beam-monitoring X-ray spectrometer conveniently may employ thin-window proportional counters in the energy region from about 0.1 to 10 kev. and a thallium-doped sodium iodide scintillation detector in the region above 10 kev. Conventional filters for suppressing low-energy bremsstrahlung may be placed in the beam path before the spectrometer.

The target material may be any desired material and usually will be selected on the basis of its characteristic K X-ray energy. Low-atomic-number materials such as beryllium and carbon produce low-energy (longwavelength) K X-rays whereas high-Z materials such as tungsten and gold produce high-energy (shortwavelength) K X-rays. In general, however, the higher the selected material atomic number, the higher the maximum possible K X-ray yield but the lower the maximum possible beam purity.

The target thickness can vary over wide limits. A convenient measure of target thickness for present purposes is the electron range," which is the path length which an incident electron travels in the target in the course of slowing down from its initial energy to zero energy. In these terms, the target thickness may vary from about 0.01 to about 2.0 electron range, but for ruggedness and high K X-ray beam yield and purity, the thickness preferably varies from about 0.3 to abuot 1.0 electron range.

The target inclination angle, measured between the target normal and the electron beam axis or direction, may vary from about 0 to about For high K X-ray yields, however, the target inclination angle preferably is set from about 0 to about 45".

From extensive measurements it has been found that the electron beam energy should be selected between lower and upper limits, both of which increase with the target atomic number. These limits may be empirically expressed as (Z+0.0014Z). and (3Z+0.0092Z) kev., respectively. In general the K X-ray beam purity is maximum at the lower electron beam energy limit and decreases as the electron beam energy is increased. The K X-ray beam yield, however, is maximum at the upper electron beam energy limit and decreases as the electron beam energy is decreased.

The output beam collimation angle should range from about to about measured with respect to the electron beam direction. The K X-ray beam purity is maximum at 180 and decreases slightly as the collimation angle is decreased to about 120; beyond about 120, the purity decreases rapidly.

EXAMPLE I A thin molybdenum target (Z=42) is mounted in an evacuated chamber and bombarded at an inclination angle of about 0 with a substantially monoenergetic electron beam having an energy in the region from about (42 0.001442) to about (3.42 0.009242) kev. from about 146 to about 808 kev. FIG. I illustrates the intensities of the K X-rays and bremsstrahlung emitted from the target as a function of the observation angle, where the electron beam direction is taken as 0. For this figure, the electron beam energy is about 200 kev. and the target thickness is about 0.0! electron range. It can be seen that the K X-ray intensity is isotropic whereas the bremsstrahlung intensity is peaked in the 0 direction. The reason for this is that the K X-rays, produced by outer-shell target electrons falling into electron-excited K-shell vacancies, are

emitted isotropically, while the bremsstrahlung, produced by deflections of the incident high-velocity electrons, is predominantly emitted in the electron beam direction. As a result, an X-ray output beam taken at an angle from about 120 to about 180 has a high K X-ray beam purity, i.e. a high ratio of K X-rays to bremsstrahlung photons.

FIG. 2 illustrates an evacuable chamber 10 for housing the X-ray target 12, with an electron beam inlet tube 14 for connecting the chamber to an electron beam source such as a linear accelerator, and an X-ray output tube 16 for collimating the X-rays emitted from the target at an angle of about 120 with respect to the electron beam direction. The output tube 16 may have a vacuum-tight X-ray transmissive window and additional collimating apertures (not shown). A beam stop tube 18 may be provided opposite the electron beam inlet tube 14.

FIG. 3 shows an alternative evacuable chamber 20 for housing the X-ray target 22. The chamber includes electron-beam-deflecting means such as a U-shaped magnet 21 having spaced-apart lower and upper pole faces 23 and 25, respectively, for establishing a magnetic field across the path of the incoming electron beam. The magnetic field deflects the electron beam onto the X-ray target 22 and permits the X-rays emitted from the target at an angle of about 180 with respect to the incident electron beam direction to be taken off by the X-ray output tube 26.

EXAMPLE II Using the apparatus shown in FIG. 3, targets of different atomic numbers are bombarded with an electron beam variable from about 10 to 4,000 kev. The target thicknesses are about 0.3 to about 1.0 electron range, the inclination angle is and the X-ray beam collimation angle is 180. The K X-ray yields and beam purities are measured with an X-ray spectrometer as described above. The maximum K Xray yields and purities obtained from the targets are summarized in graph form in FIG. 4. Curves l and 2 designate the cases with corresponding electron beam energies for maximum yields and for maximum purities respectively. (The above empirical expressions for the electron beam energy limits as a function of target atomic number are derived from the bottom two curves.) FIG. 4 shows that, for each atomic number, the maximum purity is achieved at the sacrifice of reducing the yield below its maximum value. Also, the maximum attainable purity decreases as the target atomic number increases. For highatomic-number targets, the beam purity may be increased by using a thinner target, say about 0.01 to about 0.3 electron range, though the K X-ray yield will be decreased.

We claim:

1. A method of producing an intense, high-purity K X-ray beam, which comprises:

bombarding an X-ray target of material of atomic number Z with a substantially monoenergetic electron beam having an energy in the region from about (Z+0.0014Z to about (3Z+0.0092Z kev.; and

collimating into an output beam the X-rays emitted from the target at an angle in the range from about to about with respect to the direction of the electron beam.

2. The method of claim 1 wherein the thickness of the target is in the region from about 0.3 to about 1.0 electron range.

3. The method of claim 1 wherein the angle of inclination of the target with respect to the electron beam direction is in the range from about 0 to about 45.

4. The method of claim 1 and further including the step of deflecting the electron beam prior to incidence on the target to permit access to the X-rays emitted from the target at an angle of about 180 with respect to the electron beam direction. 

1. A method of producing an intense, high-purity K X-ray beam, which comprises: bombarding an X-ray target of material of atomic number Z with a substantially monoenergetic electron beam having an energy in the region from about (Z+0.0014Z3) to about (3Z+0.0092Z3) kev.; and collimating into an output beam the X-rays emitted from the target at an angle in the range from about 120* to about 180* with respect to the direction of the electron beam.
 2. The method of claim 1 wherein the thickness of the target is in the region from about 0.3 to about 1.0 electron range.
 3. The method of claim 1 wherein the angle of inclination of the target with respect to the electron beam direction is in the range from about 0* to about 45*.
 4. The method of claim 1 and further including the step of deflecting the electron beam prior to incidence on the target to permit access to the X-rays emitted from the target at an angle of about 180* with respect to the electron beam direction. 